The invention relates to a differential gear unit for motor vehicles, comprising a housing and, therein, a differential gear mechanism, two coaxial gear mechanisms and two controllable friction clutches, the differential gear mechanism comprising an element which can be driven (for example, a differential cage) and a first and second output element, for example the axle shafts, the coaxial gear mechanisms transmitting an additional torque to the first and second output elements according to the position of the friction clutches.
Units of this type permit active motive power distribution control between the two output elements. The coaxial gear mechanism brings about a step-up or a step-down transmission ratio, with the result that the additional motive power can be metered via the controllable friction clutch to the respective output elements depending on the driving situation. Experts call this “torque vectoring”.
A differential gear unit of the generic type is known from U.S. Pat. No. 5,370,588. In said differential gear unit, the coaxial gear units are equipped with double planets instead of internal gears, and are connected fixedly in terms of rotation to in each case one side of the clutch. This type of construction requires a considerable installation width, which is unfavorable, however, in particular if homokinetic joints adjoin the axle shafts. The coaxial gear mechanism is a fixed-axle gear mechanism, the input and output elements of which are coaxial.
Furthermore, a differential gear unit of this type comprises a very large number of components which are difficult to mount and are loaded highly, and the lubrication/cooling of the clutch which is accommodated deep in the interior of the housing is critical. In the case of insufficient cooling, the friction clutch cannot be operated with slip for a relatively long time. Its use value is therefore greatly limited.